by Elizabeth Weise and Liz Szabo, USA TODAY

by Elizabeth Weise and Liz Szabo, USA TODAY

An earlier version of this story misstated the method used to inactivate the flu virus in injected flu vaccines. The virus is rendered inactive by chemicals.

There's a belief, popular in the current flu outbreak, that getting vaccinated can actually give you the flu, and many people use it as a reason to avoid the shot. A survey by CVS Pharmacy last year found that about 35% of consumers think it's true.

Doctors say it's impossible.

While some people get sick after being vaccinated, it's not from the vaccine, doctors say. The most common side effect is a sore arm from the shot.

Vaccines in flu shots use killed viruses, also called inactivated vaccines.

For the influenza vaccine, chemical inactivation is used to 'kill', or make the virus inactive and noninfectious, says Charles Chiu, a professor of infectious disease at the University of California-San Francisco. The chemicals leave enough of the outer coating of the virus intact that it "elicits a more robust and reliable immune response" in the body but is entirely incapable of reproducing, says Chiu.

Enough of the proteins on the virus' shell are left intact to trigger an immune response. The body recognizes the proteins as a danger and produces white blood cells to attack any matching flu virus, but those proteins aren't enough for the virus to reproduce and infect a person who's been vaccinated.

Some people develop mild body aches, fatigue, muscle pain and a low fever, "but it's very rare and it's not due to flu. It's the body's immune response" kicking in, Chiu says. The symptoms are "very mild and typically go away in a day or so."

Another type of vaccine, called attenuated or live virus vaccine, is made from highly weakened virus. The FluMist nasal spray vaccine, popular for children, contains such a weakened virus..

"These viruses are grown so that they're cold-adapted, meaning they can cause infection only at the cooler temperatures found in the nose," Chiu says. "They can't survive in the rest of the body. It's too hot."

The most common side effects of FluMist are sniffles or congestion, caused by the virus reproducing in the nose. That reproduction helps to stimulate the immune system to make antibodies against the virus. These symptoms "only last a day or two and then they disappear," said Chiu.

Chiu thinks people who believe the vaccine gave them the flu may never have had the real flu. If they had, he says, they'd know it's nothing mild and not the sort of thing that allows you to go to work with the sniffles.

"It knocks you out," he says. "It's a systemic illness that includes not only sneezing and coughing but very high fever and extreme muscle and body aches."

There are several reasons why people link getting a flu shot with getting sick. Most likely is that the person who got vaccinated came down with a different kind of respiratory virus, not the flu.

"There are hundreds of these that circulate each winter," says Alicia Fry, a medical epidemiologist in the influenza division at the Centers for Disease Control and Prevention in Atlanta. It's also possible that the immune system hadn't had time to fully arm itself against the flu after being vaccinated, a process that takes about 14 days.

Although the flu vaccine can't give you the flu, it is possible to get the flu after you've been vaccinated. This year's vaccine protects against three flu strains, H2N3, H1N1 and influenza B, and is a 90% match for the viruses circulating this winter. If you have the misfortune of being exposed to a different flu strain, the vaccine can't protect you, says Michael Jhung, a medical officer at the CDC.

In some cases, vaccine "failure" is not really the vaccine's fault, says Gregory Poland, a professor of infectious diseases at the Mayo Clinic in Minnesota who is an adviser to the CDC. Flu vaccines must be refrigerated according to exact specifications. If they freeze, or get too hot, they won't work properly, he says.

This year's flu vaccine is about 62% effective against the currently circulating strains, according to the CDC, which is in line with vaccine effectiveness for most years. The FluMist nasal vaccine is more protective, at least in children, says Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. It protects 83% of children under 8.

Don't let the odds stop you from getting vaccinated, doctors say. Not getting vaccinated means you have 0% protection.

"Vaccination is the single most important step you can take to protect yourself," Tom Frieden, director of the CDC, said last week as flu cases surged. "If you've gotten the flu vaccine, you're 62% less likely to need to go to the doctor to get treated for the flu."